Reaction injection molding (RIM) is a low pressure one-step or one-shot injection of liquid components into a closed mold where rapid polymerization occurs resulting in a molded plastic product. In a RIM process, viscosity of the materials fed to a mold is about 50 to 10,000 cps, preferably about 1500 cps, at injection temperatures varying from room temperature for urethanes to about 150.degree. C. for lactams. Mold temperatures in a RIM process are in the range of about 100.degree. to 200.degree. C. and pressures in the mold are generally in the range of about 50 to 150 psi. At least one component in the RIM formulation is a monomer that is polymerized to a polymer in the mold. In RIM, a chemical reaction takes place in the mold to transform a monomer to a polymeric state. For practical purposes, the chemical reaction must take place rapidly-in less than about 2 minutes for smaller items.
Although urethanes are the only commercial materials currently available for RIM processing, systems based on the use of nylons are being developed due to serious disadvantages of the urethane systems. A significant advantage of the nylon systems over the urethanes resides in the fact that in the nylon systems, there is no need for strictly controlling the mixing ratio of the reactive materials.
Polymerization of a lactam to give a polyamide has been known for many years. The earliest processes for this polymerization were slow, requiring several hours, and involved the use of water or acidic reagents as polymerization catalysts. Subsequent work showed that anhydrous lactams could be polymerized at about 200.degree. C. in the presence of strongly basic materials, particularly the alkali and alkaline earth metals, their hydrides, hydroxides, alkoxides, oxides, alkyls or amides. More recently, it has been disclosed that the base-catalyzed polymerization of a lactam can be accelerated by the addition of certain compounds that function as promoters. Particularly effective promoters which have been disclosed include acylating agents such as acyl halides, anhydrides and the like; isocyanates and compounds containing tertiary nitrogen having at least two of the three substituents on the nitrogen atom consisting of carbonyl, thiocarbonyl, sulfonyl, phosphenyl, thiophosphenyl and nitroso radicals.
Carrying out the RIM process normally involves the metering into a mixing head containing the promoter. The two streams are mixed in the mixing head and then conveyed into a preheated mold where polymerization takes place and where the monomer mixture is converted into a solid object. Such a process, therefore, requires critical metering of two reactive streams and the use of high speed impinging mixing heads which limit the type of fillers that can be admixed in the molding compositions.
A companion patent application entitled "RIM Process Using Single Component Composition" filed on behalf of inventor W. Yang, describes a process whereby a single monomer mixture containing a catalyst and a promoter combination is prepared in a tank. Since this mixture has a long pot life and a short mold time, it can be held in a holding tank and then conveyed directly to a preheated mold without any mixing device between the holding tank and the mold. The monomer mixture contains, inter alia, a lactam monomer or a mixture of such monomers.
The process described above, however, has a number of disadvantages due to its use of the lactam monomers. Since melting points of lactam monomers are substantially above room temperature, they have to be heated above their melting points in order to admix other ingredients. For this reason, all of the process equipment, including all transfer lines, must be heated and maintained at a temperature above the melting point of the lactam monomer to avoid solidification thereof and the consequent plugging of the equipment. Also, heating the lactam monomer to or above its melting point shortens the shelf life of the molding system and thereby lessens the possibility for the system to be processed as one component RIM.